Self-tapping threaded bushings

ABSTRACT

The disclosure relates to a threaded bushing which, upon entering and turning in the initially smooth bore of the work piece, will form through its own male thread the corresponding internal thread within the wall of the work piece bore. The bushing is formed with one or more exterior bevels or grooves which extend axially over at least several threads and intersect such threads to form spoon or cup-shaped frontal areas where each bevel or groove merges into a respective thread.

This is a continuation of Ser. No. 125,260, filed Mar. 17, 1971, and nowabandoned, which was a division of application Ser. No. 733,562, filedMay 13, 1968, and now U.S. Pat. No. 3,597,781, issued Aug. 10, 1971.

BACKGROUND OF THE INVENTION

Threaded bushings possessing a self-tapping male thread are usuallymanufactured from metals, such as steel, bronze or brass, and serve tofasten screws, bolts or similar connecting elements within or tostructural members or work pieces which have a lesser hardness than thebushing material and can consist for example of soft steel or iron,wood, plastic, light metals and the like. It is the purpose of suchthreaded bushings, also called liners, to create within a relativelysoft work piece an internal thread possessing relatively great strength.This strength is attained not only by this thread of substantiallygreater diameter relative to the internal thread diameter of the bushingbut also by avoidance of a severance of the fibers of the material, andit is one of the purposes of the invention to avoid this disadvantage.

The invention is designed to solve several problems:

1. The internal thread, produced within the bore of the work piece, isto be produced partially without cutting; for example, by notching, andpartially by cutting, i.e., by metal removal;

2. The entering torque of the threaded bushing is to be kept relativelylow, while the return torque or restoring moment should be relativelyhigh;

3. The costs for manufacturing the threaded bushing should be as low aspossible.

Self-tapping threaded bushings of various types and designs are known.For example, there exist threaded bushings which provide, at least overa portion of the length of the bushing, grooves or slots which runsubstantially parallel to its axis and which form cutting edges togetherwith the webs of the thread, and with the outside of the penetrating endof the bushing usually designed conically. These slots or grooves of theknown threaded bushings are usually produced, following the manufactureof the male thread of the bushing, by milling, grinding or the like. Themethod of manufacture is very costly. Furthermore, threaded bushings ofthis type will produce the internal thread within the bore of the workpiece almost exclusively by cutting. The forming of the thread bycutting in the manner described has the disadvantage that the fibers ofthe work piece material are severed, thereby lowering its stability andespecially its shearing strength.

SUMMARY OF THE INVENTION

In order to solve the above-described problems, the invention is basedon a threaded bushing possessing bevels, grooves, or the like whichintersect the male thread and form recesses within the originally roundcontour, and which extend axially at least over several threads butpreferably over the total axial length of the threaded bushing, andwhereby between the bevel, groove, or the like on the one hand and thethreads on the other hand (i.e., at the crests of the thread) faces orfrontal areas are created which will produce female threads within thebore of the work piece, the novel and inventive feature being that thefrontal areas of each thread are bent or curved up, relative to theoriginal contour, and are formed concavely, similar to a shovel orspoon.

Particularly significant in connection with the threaded bushingproposed by the invention is the point that the frontal areas of thethread, facing each other from both sides of a bevel, are bent or curvedup at dissimilar magnitudes relative to the original contour, and thatthey are designed in the shape of a shovel or spoon.

Another specific characteristic, significant in connection with theinvention, of these frontal areas is the fact that they possess edgeswhich are designed in the form of not too sharply edged cutting edges,or edges which will help to increase the restoring moment. These frontalareas are produced by a male thread-rolling process and appropriatedeformation of material; that is, they are formed by raising from theoriginal surface or contour of the bevel or the like.

According to the invention, this is accomplished by furnishing a profilebar which has several axially extending bevels distributed around itscontour (the bevels preferably being axially parallel), with a malethread by means of rolling tools, whereby the characteristics of therolling process, such as rolling pressure, rolling depth, rollingadvance, shape and material of the rolling tool, and the like, are setand selected in such manner that the frontal area, created in the zoneof transition between an individual crest of the thread and anindividual bevel, is curved at the thread end to a greater or lesserdegree relative to the original plane or contour of the bevel, or israised due to the deformation of material, and that the edges of saidfrontal areas project above the areas with the edges not too sharplyedged. The not too sharply edged design is a decisive feature for thecreation of a powerful restoring moment, but it leads also to the otherdesired effect, namely that upon entering and turning of the bushing theinternal thread of the work piece is being formed only partially bycutting but mainly by deformation without cutting.

Preferred basic material to be utilized is profile material, for examplebars or tubes, produced by drawing. Such profile material will alreadypossess bevels in flat, convex or concave form.

The invention proposes further that during or after the rolling of themale thread, there are produced in a manner known per se, adjacent toone or both ends of the threaded bushing, conically beveled male threadprofiles, their outer diameters tapering off toward the ends. By makingthe threaded bushing pointed at both ends, the invention makes itfeasible to select one or the other of a threaded bushing to be enteredand turned in the smooth bore of a work piece, or to fasten it withinthe work piece, utilizing the greater or lesser curved frontal area ascutting surface, or the lesser or greater curved frontal area as meansto increase the restoring torque.

The direction of rolling at the creation of the male thread of thebushing and the direction of turning of the bushing into the work piecewill either coincide or be opposite to each other, and accordingly themore heavily curved frontal area will act as thread-forming means, andthe lesser curved frontal area as means to increase the restoringtorque, or vice-versa.

The invention proposes further that the boring of the threaded bushingand the furnishing of the female thread is accomplished in a mannerknown per se either before or after the thread-rolling.

If applicable, it is further proposed that in case of a threaded bushingmade of steel the male thread or the entire bushing is tempered afterthe thread-rolling.

It has been found that such concave or spoon-shaped designs of thefrontal areas which are curved or drawn up relative to the bevel,groove, or the like possess edges which have, or can perform, at leastpartially, the function of cutting and also simultaneously of displacingmaterial if these frontal areas are utilized as internalthread-generating means.

At each bevel, groove, or the like, each individual crest of a threadpossesses two frontal areas, and according to the invention the secondarea formed in the direction of rolling has a lesser pitch than thefrontal area designed for cutting, but even the latter is still keptsufficiently concave or spoon-shaped in order to attain a restoringtorque which is greatly improved over the designs known heretofore. Itis also possible, under special circumstances, to interchange thefunctions of the two frontal areas so that the flatter area will act asthread-forming means when entering and turning in the threaded bushing,with the more pronouncedly curved frontal area increasing the exittorque during return movement.

Manufacture is acccomplished by a process covered by the invention,preferably by thread-rolling. It was found unxpectedly and surprisinglythat in case of basic material possessing the bevels or grooves at fulllength, and proper selection of the rolling surface pressure as well asthe other characteristics of the rolling process, there are created bythe thread-rolling curved frontal areas at the crests of the thread,that is within the zone of transition between the crest of the threadand the bevel or groove, a significant feature being that through suchrolling process, correctly set and controlled, said raised edges of thefrontal area are produced. The direction of rolling should correspond tothe direction of turning in of the bushing if it is desired to use thesteeper or more pronouncedly curved three-forming element (the frontalareas) for the creation of the internal thread within the smooth bore ofthe work piece; if it is desired, in case of special circumstances, toemploy the lesser curved frontal areas as the thread-forming areas, theopposite rolling direction will be chosen. As ready mentioned, in thelatter case the restoring moment is thus increased still further.

The ratio between the internal thread forming by cutting and by shapingwithout cutting will vary, depending for example on the steepness orheight of the frontal area curvature, the sharpness of the edges of thefrontal area, and other characteristics. In any event, the inventionwill have the effect of attaining a sufficient deformation of materialwithout cutting, following, timewise, the cutting phase of thethread-forming process, so that compacting of the material as well as animprovement in the fiber structure of the material of the work piece canbe attained. Such deformation of the material has the additionaladvantage that the inserted threaded bushing will be seated morestrongly and securely in the work piece.

If the edges of the frontal areas are not especially sharp, as can bedefinitely accomplished by means of a properly controlled rollingprocess, any danger of separating the fibers of the work piece materialwill be eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

Specifically, the drawings illustrate:

In FIG. 1, the profile of a solid rod with three uniformly-spaced bevelsin longitudinal direction;

In FIG. 2, a solid rod in profile with slightly convex bevels running ina longitudinal direction;

In FIG. 3, a solid rod in profile with three uniformly-spaced, slightlyconcave bevels running in longitudinal direction;

In FIG. 4, a plan view of a completed threaded bushing;

In FIG. 5, a plan view of another species of a completed bushing;

In FIG. 6, a completed threaded bushing in perspective;

In FIG. 7, an enlargement of the area VII depicted in FIG. 6;

In FIG. 8, a section along line VIII--VIII of FIG. 7;

In FIG. 9, a section along line IX--IX of FIG. 7;

In FIG. 10, a section along line X--X of FIG. 7; and

In FIG. 11, the insertion and turning process of a bushing into a workpiece.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIG. 1 shows rod 1, originally a round bar, provided with flat bevels 2,spaced uniformly at the contour and running axially parallel, that is,in a longitudinal direction of the rod.

FIG. 2 shows a substantially identical rod 1', its bevels 2' shapedslightly in convex form, but otherwise identical with bevels 2.

In FIG. 3 the bevels 2" are shaped concavely at the otherwise identicalrod 1", running again in longitudinal direction, axially parallel. Atarea 3 it is indicated that the rod can be hollowed, whereby the innerwall 3 can be provided with the required internal thread at the properstage of the manufacturing process. Usually, solid rods will bepreferable in view of the rolling process because there exists thedanger that a thin-walled tube could be deformed excessively due to therolling pressure.

FIG. 4 shows a completed threaded bushing B in plan view, where thedirection of turning is identical with the direction of rolling, bothrunning in clockwise direction. The rolling direction illustrated,creating the male thread, causes the above-discussed bulging, that is,the formation of curved-up or bent-up cutting edges 13, created byclockwise displacement of material. The rolling process also has thepeculiar consequence that there will rise in the other direction, thatis, opposite to the rolling direction, a less pronounced curvature at14, which in this case represents the restoring edge.

In case of the threaded bushing B', shown in FIG. 5 in the form of anaxial view, the rolling had been performed in opposite direction, andthis figure illustrates that the position of the edges or frontal areas13 and 14 becomes reversed, the areas being designated by numerals 13'and 14'.

FIG. 6 shows the tapering of the lower threads within area 12, a featurewhich is known per se and which can be accomplished for example byproper turning or grinding.

We wish to mention that the preferred basic material is a rod,manufactured by drawing and formed as shown in FIGS. 1-3. A significantfeature of the invention is the fact that the frontal areas 13 as wellas the opposite areas 14, created at the intersections between thebevels 2, the flanks 15a and 15b and the crests 15 of each thread at thezone of transition toward the bevel 2 or the like, have a concave shape,similar to a spoon, for example, an internal surface in the form of aspade or scoop; in other words, areas are curved inwards spherically andcup-like. Also, the roots 11 between successive threads are essentiallydiscontinuous at bevel 2.

FIGS. 7 and 8 illustrate these points in a particularly clear manner.The frontal areas 13 possess a spherical concavity of the frontal edgeor area of the crest 15 of the thread. The frontal areas 14 are shapedin the same manner although less pronouncedly. The edges 13' and 13" ofsaid frontal areas 13 are formed at the intersection of flanks 15a and15b with bevel 2 and are upwardly forced, narrow crests which act in acertain manner as cutting edges, that is, metal-removing edges; however,since they do not possess the sharpness of a knife, they will fulfillthe desired effect of not only removing metal by cutting but to begin,and at least carry out partially, the process of deforming materialduring the formation of the female thread within the work piece.Likewise, the edges or rims 14' and 14" of the frontal areas 14 willform not too sharp cutting edges, so that these frontal areas 14 willincrease significantly the restoring moment during the back-out,assuming that the direction of entry and turning corresponds to thearrow E in FIG. 7. Obviously, in case of the opposite direction ofturning, as indicated by arrow E', the frontal area 14 will be the areaforming the female thread, and areas 13 the back-out areas, causing aparticularly great increase in the restoring moment.

FIGS. 9 and 10 illustrate the sectional views along the lines depictedin FIG. 7. It should be noted that within the lower region 12 (see FIG.6), the crests of the thread are flattened, that is, designed in taperedshape, as indicated by the broken line 17 in FIG. 10 (note shown in FIG.9).

FIG. 11 shows how a threaded bushing B is introduced and turned in workpiece 19 in direction E by a turning tool 18 of known construction, thework piece possessing a known smooth bore 20, the bore diameter beingsmaller than the widest part of the outer diameter of bushing B. Thebushing B, illustrated in FIG. 11, has for example the thread-formingfrontal areas 13 and the back-out areas 14, as shown in FIG. 6.

According to a preferred embodiment, the bushings of the presentinvention are made from a length of rod of basic circular cross sectionwhich has been provided with longitudinal planar bevels generally asshown, for example, in FIG. 1. As shown in FIGS. 4, 5, 7, and 8, thelongitudinal planar bevels are located on chords of the basic circularcross section of the rod which are at a depth within the circular crosssection sufficient to ensure that subsequent thread-rolling on thecircular cross section does not produce any threads whose roots extendcompletely across the bevelled portions. In other words, the thread rootradius is chosen to be at least as large as the minimum radius betweenthe center of the circular cross section and the planar bevel. Threadrolling is properly carried out by the well-known means of three rollersspaced circumferentially about the rod, and preferably the direction ofthread rolling is clockwise, that is, in the direction of turning of thebushing into the work piece. The thread rolling rollers are adjusted asrequired by the particular thread geometry and the rod material so thatthe conventional thread rolling surfaces do not roll any thread rootscompletely across the area in which the planar bevels are located on therod, in a manner that will be appreciated by those skilled in thethread-rolling art. After the threads have been rolled onto the rod, thethen-threaded rod is cut into a plurality of pieces each of whichcomprises a single bushing.

Also covered by the invention is the manufacture of threaded bushingspossessing one or more of the above discussed features in that mannerthat first pieces of bushing length are cut off from a rod or tubesection possessing a round contour, the pieces are then provided withbevels (2,2') and with a male thread by rolling, the internal threadbeing created in usual manner.

Finally, another manufacturing process within the framework of theinvention could utilize pressure die casting with the pressure diecasting tools or molds being shaped and designed in such manner that theabove-discussed physical features, in particular the scoop-shaped formof the frontal areas 13, 14, will be created.

Having described an improved self-tapping bushing as one improvement ofthis invention, we desire it to be understood that various othermodifications and alterations may be made departing the specific formsshown without departing from the scope of the invention.

What we claim is:
 1. A threaded self-tapping bushing adapted to besecured to a work piece and comprising,an elongate member having helicalmale threads extending over at least a portion of its axial length, saidthreads having crests, roots between said crests, and flanks extendingfrom said crests to said roots on either side of said crests; saidmember having at least one axially extending bevel intersecting aplurality of successive threads, the crests and flanks of each said malethread at their intersection with said bevel defining frontal areas oneither side of said bevel which are concave both along thecircumferential direction of each said male thread and transverselythereto along the axial direction of the bushing, each said bevel beingapproximately as deep as the roots between the crests to ensure that theroots between successive crests are essentially discontinuous at eachbeveled portion, each said frontal area providing cutting edges along atleast a part of its periphery as defined by said crest, flanks, andbevel, one of the two said frontal areas formed at the junction of eachsaid male thread with said bevel having a greater curvature than theother.
 2. The threaded bushing of claim 1 in which the leading frontalarea in the direction of turning said bushing into said work piece hasless curvature than the lagging frontal area.
 3. The threaded bushing ofclaim 1 in which the beveled male thread profiles at least one end ofthe bushing are conically tapered inwardly.